1. Field of the Invention
This invention relates to optical amplifier systems and more particularly, to systems including distributed and discrete Raman amplifiers.
2. Technical Background
Long distance communication systems typically use erbium doped fiber amplifiers between long spans of transmission fiber. A typical configuration for the erbium doped fiber amplifier includes one or more pump lasers operating at wavelengths of 980 nm or 1480 nm and providing an output coupled into the erbium doped fiber. Erbium doped fiber amplifiers (EDFAs) are discrete amplifiers. Such amplifiers are used to provide sufficient gain to compensate for loss of signal in transmission fiber, often requiring multiple high power pumps. Erbium doped fiber amplifiers are used to produce high power out-going signal because, otherwise, as the signal travels through the transmission fiber, the attenuated signal level would approach the noise level by the time it reaches the next amplifier. Erbium doped fiber amplifiers typically have to be positioned at about one hundred kilometers or less from one another, otherwise the signal level would drop down to approximately the noise level before reaching the next EDFA and the next amplifier might not be able to distinguish between the noise and the signal.
Discrete Raman fiber amplifiers can be used as amplifying devices that compensate for losses incurred by the signal during its transmission through the transmission fiber. For this purpose, the discrete Raman fiber amplifiers would also be situated between long lengths (typically 40-100 kilometers) of transmission fiber. Unfortunately, an amplifier system with only discrete Raman amplifiers suffers from MPI (multipass interference), from double Rayleigh backscattering, and gain saturation from the pump depletion.
Distributed Raman fiber amplifiers are sometimes used in conjunction with the Erbium doped fiber amplifiers. The typical distributed Raman fiber amplifiers utilize typical transmission fiber as the gain medium. However, the signal propagating through the distributed Raman amplifier undergoes distortion and, thus becomes broader due to chromatic effects produced by the fiber. This distorted signal is provided to and is amplified by the Erbium doped amplifier and, contributes to reduction of the signal to noise ratio. In order to compensate for the signal broadening, such an amplifier system typically utilizes a multistage Erbium doped fiber amplifier, which has a dispersion compensating fiber between the two amplifying stages of the Erbium-doped amplifier. Splicing a dispersion compensating fiber between two stages of the Erbium doped fiber amplifier introduces about 10 dB loss into the amplifier, which may be overcome by additional pumping power with a resulting increase in cost.
According to one aspect of the present invention an amplifier system includes: (i) a distributed Raman fiber amplifier and; (ii) a discrete Raman fiber amplifier that includes dispersion compensated fiber. The discrete Raman fiber amplifier is operatively connected to the distributed Raman fiber amplifier and amplifies signals received from the distributed Raman fiber amplifier. In one embodiment, at least one source of pump signal is coupled to the distributed and to the discrete Raman fiber amplifier. The distributed Raman fiber amplifier and the discrete Raman fiber amplifier in this embodiment share optical pump power provided by the shared pump. In one embodiment of the present invention an Erbium doped fiber amplifier (EDFA) is operatively connected to a discrete Raman fiber amplifier and the Erbium dope fiber amplifier amplifies signals received from the discrete Raman fiber amplifier.
It is an advantage of the amplifier system of the present invention to have no or only minimal signal broadening without center stage a dispersion compensation fiber between EDFA stages. The loss, which would have been attributable to a dispersion amplifier, can instead be allocated to other components, such as Add/Drop multiplexer, polarization mode dispersion compensator, or dynamic gain equalization components.
It is another advantage of this invention that the optical amplifiers can be spaced at more than one hundred kilometers away from one another because the transmission losses are minimized due to use of distributed Raman fiber amplifiers.
For a more complete understanding of the invention, its objects and advantages refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows.